1. Field of the Invention
The present invention relates generally to thin-film magnetic heads for use in magnetic disk apparatus, and more particularly to such a thin-film magnetic head having four or more conductive coil layers.
2. Description of the Prior Art
In order to meet the requirement of high-density recording of information, the track width of a magnetic head is being narrowed increasingly. On the other hand, in response to the track width being narrowed to decrease the output, the thin-film magnetic head is arranged so as to be equipped with a plurality of conductive coil layers to increase the number of turns in the winding to enlarge its output and to be provided with a center tap for the winding to counter increase in the inductance accompanying the increase in the number of turns. For instance, as illustrated in FIG. 9, such a thin-film magnetic head is arranged such that insulating layers 3 made of an organic insulating material and conductive coil layers 4 are provided between a lower magnetic pole 1 and an upper magnetic pole 2 to be alternately piled up so as to form a laminated body which is placed on a base 6 with a gap member 5 being interposed therebetween.
A further description will be made hereinbelow in terms of the insulating layers 3 and the conductive coil layers 4. On the upper surface of the gap member 5 there is placed the first insulating layer 3a, and on the upper surface of the first insulating layer 3a there is disposed the first conductive coil layer 4a. Further, successively piled up are the second insulating layer 3b, second conductive coil layer 4b, third insulating layer 3c, third conductive coil layer 4c, fourth insulating layer 3d, fourth conductive coil layer 4d and fifth insulating layer 3e. The lower magnetic pole 1, conductive coil layers 4a to 4d and upper magnetic pole 2 are respectively insulated by means of the first to fifth insulating layers 3a to 3e. In addition, although not illustrated, the first conductive coil layer 4a and second conductive coil layer 4b, the second conductive coil layer 4b and third conductive coil layer 4c, and the third conductive coil layer 4 c and fourth conductive coil layer 4d are at their end portions coupled to each other, respectively. Moreover, a center tap is drawn from the connection point between the second and third conductive coil layers 4b and 4c, and lead lines are respectively provided at the other end portions of the first and fourth conductive coil layers 4a and 4d.
Here, for the thin-film magnetic head having the center tap, since two coil portions divided by the center tap are required to be coincident in resistance and inductance with each other (see the Japanese Patent Laid-Open Publication Nos. 61-255523 and 63-63114), the conductive coil section comprising the first and second conductive coil layers 4a, 4b and the conductive coil section comprising the third and fourth conductive coil layers 4c, 4d are generally arranged so as to be equal in the number of turns in the coil and the width of the coil to each other. In addition, when piling up the respective conductive coil layers, a height difference appears with respect to the circumference, and because limitation is imposed upon the mask engraving on manufacturing, the respective peripheral ends of the conductive coil layers 4a to 4d and the insulating layers 3a to 3e are required to be successively reduced from the lower layer to the upper layer so as to incline the circumferential surface as a whole, thus more decreasing the coil pitch of the conductive coil layer as the position becomes higher.
There is a problem which arises such a conventional thin-film magnetic head, however, in that, as the number of the coil layers increases, the height difference increasingly enlarges with respect to the peripheral portions so as to make severer the engraving condition. Moreover, the coil pitch become smaller as the position becomes higher to reduce the space between the coils. This also make more difficult the engraving.
Although the Japanese Patent Laid-Open Publication No. 63-63114 discloses a multilayer balance-winding thin-film magnetic head where the respective coil layers are formed by a bifilar and a midpoint terminal is provided so that two coil sections are coincident in the number of turns and the width of the coil with each other and steps disappears at the circumferential end portions of the coil layer and the insulating layer, since in this magnetic head two conductive coils are simultaneously provided in parallel to each other by the bifilar, the outside length becomes greater than the inside length and hence the resistance of the outside conductive coil section becomes higher, whereby the electromagnetic characteristics of the two conductive coil sections divided by the midpoint terminal becomes unbalanced.